1. Field of the Invention
The present invention relates to a process for the preparation of microcapsules by polycondensation of a melamine/formaldehyde resin mixture in water, in which the material forming the capsule core is dispersed, in the presence of an anionic protective colloid, and the melamine/formaldehyde resin mixtures used therein. The present invention furthermore relates to the microcapsules obtained by the novel process and to their use for pressure-sensitive recording systems.
2. Description of the Background
Microcapsules which are used for pressure-sensitive recording materials must be impermeable and must also have a narrow size distribution, since otherwise, when they are further processed, deficiencies occur in the copying papers. The encapsulation process and the starting materials therefore have to meet high requirements.
As a rule, partially methylated resins of 1 mol of melamine and from 5 to 6 mol of formaldehyde are used as starting materials for the wall material since they lead to particularly impermeable and uniform capsules.
However, such resins have a relatively high residual formaldehyde content, which it is intended to avoid since the microcapsules are often prepared directly by the paper maker.
Melamine/formaldehyde resins converted to the methyl ether form and prepared with a relatively low formaldehyde content are disclosed in EP-A-0 008 683 for the preparation of coatings. When used as a starting material for microcapsules, however, they are unsatisfactory since they lead to nonuniform capsule walls.